1. Field
The present invention relates generally to data communication, and more specifically to a novel and improved reverse link architecture for a wireless communication system.
2. Background
Wireless communication systems are widely deployed to provide various types of communication including voice and packet data services. These systems may be based on code division multiple access (CDMA), time division multiple access (TDMA), or some other modulation techniques. CDMA systems may provide certain advantages over other types of system, including increased system capacity.
In a wireless communication system, a user with a remote terminal (e.g., a cellular phone) communicates with another user through transmissions on the forward and reverse links via one or more base stations. The forward link (i.e., downlink) refers to transmission from the base station to the user terminal, and the reverse link (i.e., uplink) refers to transmission from the user terminal to the base station. The forward and reverse links are typically allocated different frequencies, a method called frequency division multiplexing (FDM).
The characteristics of packet data transmission on the forward and reverse links are typically very different. On the forward link, the base station usually knows whether or not it has data to transmit, the amount of data, and the identity of the recipient remote terminals. The base station may further be provided with the “efficiency” achieved by each recipient remote terminal, which may be quantified as the amount of transmit power needed per bit. Based on the known information, the base station may be able to efficiently schedule data transmissions to the remote terminals at the times and data rates selected to achieve the desired performance.
On the reverse link, the base station typically does not know a priori which remote terminals have packet data to transmit, or how much. The base station is typically aware of each received remote terminal's efficiency, which may be quantified by the energy-per-bit-to-total-noise-plus-interface ratio, Ec/(No+Io), needed at the base station to correctly receive a data transmission. The base station may then allocate resources to the remote terminals whenever requested and as available.
Because of uncertainty in user demands, the usage on the reverse link may fluctuate widely. If many remote terminals transmit at the same time, high interference is generated at the base station. The transmit power from the remote terminals would need to be increased to maintain the target Ec/(No+Io), which would then result in higher levels of interference. If the transmit power is further increased in this manner, a “black out” may ultimately result and the transmissions from all or a large percentage of the remote terminals may not be properly received. This is due to the remote terminal not being able to transmit at sufficient power to close the link to the base station.
In a CDMA system, the channel loading on the reverse link is often characterized by what is referred to as the “rise-over-thermal”. The rise-over-thermal is the ratio of the total received power at a base station receiver to the power of the thermal noise. Based on theoretical capacity calculations for a CDMA reverse link, there is a theoretical curve that shows the rise-over-thermal increasing with loading. The loading at which the rise-over-thermal is infinite is often referred to as the “pole”. A loading that has a rise-over-thermal of 3 dB corresponds to a loading of about 50%, or about half of the number of users that can be supported when at the pole. As the number of users increases and as the data rates of the users increase, the loading becomes higher. Correspondingly, as the loading increases, the amount of power that a remote terminal must transmit increases. The rise-over-thermal and channel loading are described in further detail by A. J. Viterbi in “CDMA: Principles of Spread Spectrum Communication,” Addison-Wesley Wireless Communications Series, May 1995, ISBN: 0201633744, which is incorporated herein by reference.
The Viterbi reference provides classical equations that show the relationship between the rise-over-thermal, the number of users, and the data rates of the users. The equations also show that there is greater capacity (in bits/second) if a few users transmit at a high rate than a larger number of users transmit at a higher rate. This is due to the interference between transmitting users.
In a typical CDMA system, many users' data rates are continuously changing. For example, in an IS-95 or cdma2000 system, a voice user typically transmits at one of four rates, corresponding to the voice activity at the remote terminal, as described in U.S. Pat. Nos. 5,657,420 and 5,778,338, both entitled “VARIABLE RATE VOCODER” and U.S. Pat. No. 5,742,734, entitled “ENCODING RATE SELECTION IN A VARIABLE RATE VOCODER”. Similarly, many data users are continually varying their data rates. All this creates a considerable amount of variation in the amount of data being transmitted simultaneously, and hence a considerable variation in the rise-over-thermal.
As can be seen from the above, there is a need in the art for a reverse link channel structure capable of achieving high performance for packet data transmission, and which takes into consideration the data transmission characteristics of the reverse links.